This invention relates to apparatus for producing a reversible electric current. More particularly, it is concerned with apparatus for producing a reversible current to provide inaudible metering pulses on a two-wire telephone trunk circuit.
In telephone communication systems it is conventional to reverse the polarity of the local battery on a two-wire trunk circuit to indicate answer supervision and also to create short duration pulses on the line for remote metering purposes. Typically these pulses are approximately 250 millisecond restorations of the idle line polarity to the circuit. Since the metering pulses are transmitted over the circuit at the same time that the circuit is being used for normal conversation, techniques must be employed to insure that they do not interfere with the normal conversation being carried by the circuit.
The usual methods of accomplishing battery reversal employ relays to switch the battery polarity applied to the circuit. In order to remove audible transients generated by the steep rise and fall times of the pulses produced by employing relays, inductor-capacitor filters are employed at the far-end trunk terminations to filter the audible transients from the voice path. The use of relays introduces certain problems in systems of the foregoing type. The lifetime of a relay is relatively short, particularly when metering pulses are applied frequently and at a high rate. In addition relays are bulky and are not readily amenable to incorporation in dense packaging arrangements.
Inductor-capacitor filter networks are also large devices, and have the disadvantage of requiring tuning in order to optimize their response to a particular trunk circuit length. Furthermore, some systems have not employed the common practice of providing filters at the far-end trunk terminations, and thus have no protection from audible transients on the circuit.